The invention described herein may be manufactured and used by or for the United States Government for governmental purposes without the payment of any royalties thereon.
The present invention relates generally to magnetic transducers, and more particularly, to magnetic sensors used with a microelectromechanical system (MEMS)-type components.
Interest is increasing in the development of miniature sensors for sensing magnetic fields in extraterrestrial, industrial, biomedical, oceanographic, and environmental applications. The trend in magnetic sensor design and development is constantly toward smaller size, lower power consumption, and lower cost for similar or improved performance.
There are several types of magnetometers (magnetic sensors with external instrumentation) currently used. The least expensive and least sensitive devices have resolution of about 10xe2x88x921 Oersted (Oe)/Hzxc2xd and typically are Hall effect devices. These devices work by sensing a voltage change across a conductor or semiconductor placed in a magnetic field. Such devices do not lend themselves for applications requiring greater sensitivity, such as that required in brain scan devices and magnetic anomaly detection devices. Flux gate magnetometers are more sensitive, having resolution of approximately 10xe2x88x926 Oe/Hzxc2xd. Flux gate magnetometers use a magnetic core surrounded by an electromagnetic coil and are difficult to microfabricate. Additionally flux gate magnetometers require relatively large amount of power and accordingly do not lend themselves to low-cost, compact, portable design. The most sensitive magnetometers called SQUIDS (superconducting quantum interference detectors) have a resolution of about 10xe2x88x9210Oe/Hzxc2xd. However, because they include a superconducting element, these apparatus must include cooling means at liquid gas temperatures, making them extremely bulky and expensive to operate. Their size limits their utility because the active superconducting element cannot be placed directly adjacent to the source of the magnetic field, for example the brain. Accordingly, there is need for small, inexpensive, low power magnetometers that have sufficient sensitivity to be useful for a variety of magnetometer applications at low frequencies. Magnetoresistive sensors are suited for low-cost, medium-sensitivity application. For example, using spin-dependent tunneling magnetoresistive sensors, one can observe 38% changes in the resistivity in fields of a few Oe, see D. Song, J. Nowak and M. Covington, J. Appl. Phys, 87, 5197 (2000).
A well known way of increasing the sensed magnetic field by a magnetic sensor is by use of a flux concentrator, which can enhance a sensed magnetic field by as much as a factor of 50, see N. Smith et al., IEEE Trans. Magn. 33, p. 3358 (1997). An example of such a device is taught in U.S. Pat. No. 5,942,895, entitled xe2x80x9cMagnetic field sensor and current and/or energy sensor,xe2x80x9d that use Hall sensors with flux concentrator components.
Magnetic sensors used to detect objects that move slowly typically possess considerable low frequency 1/f-type noise, an unwanted condition. In general, there is a tendency for such devices that have higher sensitivity to also exhibit higher 1/f-type noise. This generally occurs when using magnetoresistive-type magnetic sensors, see van de Veerdonk et al. J. Appl. Phys. 82, 6152 (1997).
A magnetic sensor (magnetometer) that addresses 1/f-type noise is taught in U.S. Pat. No. 4,864,237. This disclosure teaches of an apparatus for measuring magnetic fields, that change only at extremely low frequencies. The apparatus uses a SQUID magnetometer that includes a superconducting flux transformer that inductively couples a detected signal into a d-c SQUID sensor. This magnetometer can optionally include a device for modulating the detected signal in a frequency range characteristic of low-noise operation of the SQUID. The modulation frequencies are generally above 1 Hz and optionally even above 1-kHz. Limitations of this device include need for cryogenic operation, which inherently do not lend themselves to relatively lost cost, low power use.
Thus, there is need, for small, low-cost, low-power-consuming magnetic sensors having sensitivities capable of meeting the varied applications listed above for detecting low frequency signals and minimize 1/f-type noise.
It is an object of this invention to provide a magnetic sensor with flux concentrator having sufficient sensitivity for a variety of applications that minimize the effects of 1/f-type noise.
It is a further object of this invention to provide such a magnetic sensor with flux concentrator that is inexpensive to manufacture, having a magnetic sensor having high sensitivity, yet not having to be concerned with 1/f-type noise associated with that type of sensor.
It is a further object of this invention to provide such a magnetic sensor with a flux concentrator that uses relatively little power.
It is a further object of this invention to provide such a magnetic sensor with flux concentrator that can be readily produced by microfabrication MEMS-techniques.
This invention results from the realization that a small and inexpensive yet extremely sensitive magnetic sensor, may be accomplished by oscillating a torsionally suspended flux concentrator or a flux concentrator that can rotate or oscillate about an axis.
The invention improves the sensitivity of magnetic sensors in general that operate at low frequencies by using flux concentrators that modulate an observed sensed magnetic field with low frequency signals, thereby shifting this observed field to higher frequencies where the noise of the sensor is smaller to minimize 1/f-type noise. This is accomplished by providing a torsionally suspended microelectromechanical (MEMS)-type magnetic flux concentrator or a flux concentrator that can rotate or oscillate about some axis in combination with a magnetic sensor, preferably on a common substrate. Such a combined device is used in a magnetonmeter. Such a device is small, low-cost and has low-power-consumption requirements. The magnetic sensor can be a Hall effect or other type of magnetic sensor. At least one torsionally suspended or free to rotate MEMS-type fabricated flux concentrator is used with the magnetic sensor. The torsionally suspended flux concentrator is a flux concentrator that can rotate or oscillate about some axis at a modulation frequency much greater than an observed lower-frequency signal being sensed.